1. Field of the Invention
The invention relates to a bistable polarized electromagnet having a fixed magnetizable circuit which is excited by an associated coil through which an electric current flows with one of two possible polarities, and a mobile armature having a permanent magnet on the two opposite pole faces of which are connected two respective pole pieces, this armature being adapted to move longitudinally between two positions in each of which these pole pieces cooperate with distinct portions of a fixed circuit through two air gaps placed in series, one at least of which is variable.
2. Description of the Prior Art
Such electromagnets are widely used in industrial installations and automated systems, for example for reducing energy consumption and for ensuring that the circuits retain their state should power sources fail, and may be illustrated for example by the French Pat. No. 2358006 in which the two variable air gaps are placed in series and vary simultaneously, whereas in each of the two stable states the flux of the permanent magnet closes on a magnetic circuit with negligible reluctance. In such an electromagnet, replacement of the coil is not easy because of the presence of projecting pole pieces.
Furthermore, the presence of two air gaps placed in series means that the coil must develop a sufficiently high number of ampere-turns to oppose the flux developed by the magnet, while allowing it to flow through two reluctances placed in series; in this known apparatus, a certain technical difficulty is further met with in so far as correct simultaneous application of the pole pieces on the magnetic circuit is concerned, because of the distance which separates them.
DE-A-3 508 768 describes a polarized electromagnet having a polarized mobile piece without pole extensions, and with two sliding air gaps of constant thickness. No magnetically stable position is conferred on the mobile piece, which further requires, for its movements, the presence of two symmetric magnetizable systems, so of two coils. The mobile piece, which does not have a variable air gap, is only subjected to low tangential components of traction or repulsion forces which develop perpendicularly to the direction of movement; a limit to the movement is only provided by the meeting of feet associated with the ends of openings.
The electromagnet described in the application EP-A-179911, in the embodiment shown in FIG. 5, has a single coil (11) for causing excitation of a fixed magnetic circuit (7) having a working air gap of variable thickness (13) and a flux closure air gap (15) of constant thickness for causing the movements of a mobile armature formed by a permanent magnet (46) and by two opposite pole pieces (45).
In the structure described, the longitudinal arrangement of the polarization of the magnet requires the use of transverse pole pieces only the edges of which cooperate with the fixed circuit, so that the corresponding reluctances are high.
Furthermore, the fluxes developed by the permanent magnet in each of the two end positions of the armature (FIGS. 5a and 5b) are largely magnetically short circuited by the presence of air gap pairs which, in these positions, do not contribute to maintaining stable positions. No guide for the mobile armature is described.
In the embodiment shown in FIG. 3, the orientation of the permanent magnets is transversal, but no pole piece is associated with the permanent magnet for reducing the reluctance of the constant thickness air gap. In each of the two stable positions, the presence of a residual air gap prevents the development of a holding flux, whereas no information is given concerning the location of the guide means.